Expandable reamers are typically employed for enlarging boreholes in subterranean formations. In drilling oil, gas, and geothermal wells, casing is usually installed and cemented to prevent the well bore walls from caving into the borehole while providing requisite shoring for subsequent drilling operation to achieve greater depths. Casing is also installed to isolate different formations, to prevent cross flow of formation fluids, and to enable control of formation fluids and pressure as the borehole is drilled. To increase the depth of a previously drilled borehole, new casing is laid within and extended below the original casing. The diameter of any subsequent sections of the well may be reduced because the drill bit and any further casing must pass through the original casing. Such reductions in the borehole diameter may limit the production flow rate of oil and gas through the borehole. Accordingly, a borehole may be enlarged in diameter when installing additional casing to enable better production flow rates of hydrocarbons through the borehole.
One approach used to enlarge a borehole involves employing an extended bottom-hole assembly with a pilot drill bit at the end and a reamer assembly some distance above the pilot drill bit. This arrangement permits the use of any standard rotary drill bit type (e.g., a rolling cone bit or a fixed cutter bit), as the pilot bit and the extended nature of the assembly permit greater flexibility when passing through tight spots in the borehole as well as the opportunity to effectively stabilize the pilot drill bit so that the pilot drill bit and the following reamer will traverse the path intended for the borehole. This aspect of an extended bottom hole assembly is particularly significant in directional drilling. Expandable reamers are disclosed in, for example, U.S. Pat. No. 7,900,717, issued Mar. 8, 2011, to Radford et al., U.S. Pat. No. 8,028,767, issued Oct. 4, 2011, to Radford et al., and U.S. Patent Application Pub. No. 2011/0073371, published Mar. 31, 2011, to Radford, the disclosure of each of which is incorporated herein in its entirety by this reference. The blades in such expandable reamers are initially retracted to permit the tool to be run through the borehole on a drill string, and, once the tool has passed beyond the end of the casing, the blades are extended so the bore diameter may be increased below the casing.